The present invention relates generally to a four-unit zoom lens system having a high zoom ratio, and more particularly to a zoom lens system which is suitable for use on lens shutter cameras having a limitless back focus, etc., and which is of compact size albeit having a high zoom ratio.
So far, zoom lenses for use on lens shutter cameras are typically broken down into (1) a two-unit or positive-negative type, (2) a three-unit or positive-positive-negative type, and (3) a three-unit or negative-positive-negative type, all being now commercially available. Of these three typical versions of zoom lenses, the two-unit type (1) cannot basically have any high zoom ratio primarily because of a large fluctuation of aberrations incidental to zooming, and especially because large curvature of field at an intermediate focal length cannot well be compensated for. Moreover, an increase in the amount of movement of each unit is not preferable in camera size reduction.
On the other hand, both the three-unit types (2) and (3) can make compensation for a fluctuation of aberrations during zooming and enable the amount of movement of each unit to be reduced to some extent, so that they can have a zoom ratio higher than that of the two-unit type (1). For instance, typical examples of the three-unit types of zoom lenses having a high zoom ratio are known from JP-A 2-135312, and JP-A 2-201410. The former belongs to the three-unit type (2) with a zoom range of 38 to 135 mm, and the latter to the three-unit type (3) with a zoom range of 35 to 135 mm.
Of the three-unit types, the type with the preceding or first unit having negative refracting power, as in the case of the type (3), is not preferable for achieving compactness, because the spaces between the units reaches a maximum at the wide-angle end, resulting in increases in the overall length and outer diameter of the lens.
In contrast, the three-unit type (2) may prima facie be desired because the preceding or first unit has positive refracting power, so that reductions in the overall length and outer diameter of the lens can be achieved. Even so, it is impossible to make a camera on which this zoom lens is mounted compact, because to make the zoom ratio high results inevitably in an increase in the amount of movement of each unit.
One approach to solving this is to make the three-unit type (2) more sophisticated so as to make up for the degree of freedom of design. For instance, a four-unit type of zoom lenses including the preceding unit of positive refracting power have been put forward, as set forth in JP-A 63-43115 and JP-A 2-223908. All these are a four-unit or positive-negative-positive-negative type. More specifically, the former is a modification of the three-unit or positive-positive-negative type (2), in which the second unit is divided into negative and positive sub-units. This has a zoom ratio of about 3 that was said to be very high at the time an application was filed for that invention. The latter is a so-called double telephoto zoom type designed for use on a lens shutter camera. It is here noted that this type has been developed originally for a single-lens reflex camera. It is also noted that this preceding example has a zoom range of 38 to 135 mm.
As mentioned above, a type of zoom lens with the preceding unit having positive refracting power is desired to achieve high zoom ratios and compactness. However, the zoom lens set forth in JP-A 2-135312 makes it difficult to reduce the size of a camera on which this lens is mounted because of being of the three-unit type wherein the amount of movement of each unit becomes larger during zooming. In this connection, this zoom lens has a telephoto ratio of as high as about 1.2 at the telephoto end.
The zoom lens set forth in JP-A 63-43115 has a telephoto ratio of about 1.1 at the telephoto end. However, this is not preferable because some considerable strictness is given on the accuracy of decentration between the sub-units formed by dividing the second unit of the three-unit or positive-positive-negative type.
The zoom lens set forth in JP-A 2-223908 has a telephoto ratio of about 1.0 at the telephoto end. However, this is not preferable because the space between the second and third units reaches a maximum at the wide-angle end with the entrance pupil located farther, resulting in a large increase in the outer diameters of the lenses of the first and second units. Although the amount of movement of each unit is reduced, this is again not preferable because of an increase in the overall length of the lens at the wide-angle end.